In prior art high speed passive optical networks (H-PON), no DC optical signal is allowed in the upstream from the optical network unit (ONU) on the customer premises to the optical line terminal (OLT) located in the central office. This requires a semiconductor laser diode at the ONU to be operated without bias.
However, when a semiconductor laser diode is operated without bias in the prior art, the optical output does not turn off immediately after the driving signal is turned off. That is, a tail is observed in the optical output. Such a situation is illustrated in FIG. 1.
The tail emission is caused by 1) electrical charge that is stored in the laser diode while it is being driven and 2) the fact that the process of recombination of the charges in the laser diode is a slow one even after the driving current is removed.
Conventionally, the tail extends over 100 ns. However, in the H-PON, which is a time shared system in which multiple customer premises ONUs transmit their data in sequence in their designated time slots, the signal packets from the different customer premises ONUs often wind up being spaced much closer together than the length of the tail. This implies that a packet from a particular customer premises ONU is influenced by the tail that was generated by the customer premises ONU which was transmitting a packet in the time slot immediately precedent to that of the particular customer premises ONU.